In the radio communication, commonly the user can hold communication with the person on the other end under optimal conditions by using the antenna having the same polarization characteristic mutually. Also, if plural types of polarizations such as vertical polarization, horizontal polarization, circular polarization, and the like, for example, are used selectively as occasion demands, either a reduction in interference with other communications or an improvement in receiving sensitivity can be achieved in a variety of environments.
As the technology of the antenna unit capable of using plural types of polarizations selectively in the prior art, for example, the technologies set forth in Patent Literature 1, Patent Literature 2, and Patent Literature 3 have been known.
In Patent Literature 1, it has been proposed that a monopole antenna and a loop wire having two parallel straight portions are provided to the ground plane and then linear polarization, clockwise circular polarization, and counter clockwise circular polarization are used selectively without change of a direction or a configuration of the antenna by operating a polarization changing switch provided between them.
In Patent Literature 2, it has been proposed that a plurality of loop antennas arranged along mutually different case surfaces of the receiver unit respectively and a switching means for selecting the receiving signal are provided and then high receiving performance can be ensured by selecting the antenna that is effective in the directivity and the plane of polarization.
In Patent Literature 3, it has been proposed that a polarization diversity reception is carried out by providing a monopole antenna, which receives one of two radio waves having a different plane of polarization mutually, to the inside of a loop antenna, which receives the other of two radio waves.    Patent Literature 1: JP-A-2000-77934    Patent Literature 2: JP-A-11-88246    Patent Literature 3: JP-A-2001-332930